Page 29

3. BASICS OF WOOD DESIGN

STRENGTH OF MATERIALS

    All beams bend to some extent.  How a beam bends and how it reacts to loads depends upon the natural properties of its composition.  When the beam is loaded, the top of the beam becomes compressed (compression) and the bottom of the beam is stretched (tension).  Halfway between the tension and compression the forces are neutral.  This is called the neutral axis.  Since the upper and lower portions of a beam are providing almost all of the bending strength, an economy of materials can be achieved by eliminating material near the neutral axis.  An example is the wide flange steel beam. It is sometimes acceptable to drill holes through a beam at or near the neutral axis without affecting its bending strength.

    Materials such as concrete and masonry are very strong in compression, but weak in tension.  This is why concrete and masonry are often reinforced with steel, which is very strong in tension.  In a concrete beam the steel is added to the part of the beam which is subject to tension. This is also the reason steel is added to concrete foundations.